Determing the size of a lapping tool

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chief
Posts: 236
Joined: Sat Jan 04, 2003 5:51 pm

Determing the size of a lapping tool

Post by chief »

Hey All,

I want to make a tool to lap a 30 mm bore and I'm wondering what diameter it need to be.

Some of you probably guessed from the 30 mm, it's for rifle scope rings. The rings are steel, and the purpose of the lapping is not so much to bring them to size, but to insure the two rings are aligned with each other, and hopefully the bore of the rifle.

Is there a reference work, like Machinery's handbook or something else that would address this issue?

Terry
SteveM
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Joined: Mon Jun 27, 2005 6:18 pm
Location: Wisconsin

Re: Determing the size of a lapping tool

Post by SteveM »

Lapping removes so little material, I'm not sure that lapping is the method to use to ensure alignment.

A long reamer first might do better and then lap for final finish.

Maybe some sort of hone. The better Model A rebuilders will line hone the holes that the kingpins go thru.

Steve
RSG
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Joined: Fri Feb 04, 2011 9:59 am
Location: Ontario, Canada

Re: Determing the size of a lapping tool

Post by RSG »

My buddy turned a piece of carbon steel to 30mm x 10" and used it with lapping compound, worked well
Vision is not seeing things as they are, but as they will be.
chief
Posts: 236
Joined: Sat Jan 04, 2003 5:51 pm

Re: Determing the size of a lapping tool

Post by chief »

Steve, the rings should be fairly close as built, so not much needs to be removed. I suspect a 30 mm reamer would cost way more than i'm interested in. LoL This is a common thing for shooters, and several folks, such as Weavers and Sinclairs sell tools for the job and all of them are lapping tools.

RSG, that's my plan as well, but I have no experience with this type of lap, so I thought perhaps it needed to be a few thou undersized.

Terry
pete
Posts: 2518
Joined: Tue Feb 10, 2009 6:04 am

Re: Determing the size of a lapping tool

Post by pete »

Originally the scope ring lapping method came from the bench rest shooting crowd. It's a good idea since the scope isn't then under any bending stresses. What action and scope rings Chief? There's a few tricks to doing it, lightly clamp up the rings so the lap will still move but with almost zero looseness, the lap has to be partially rotated as well as moved length ways through the rings for each pass. After enough lapping you might need to slightly tighten the rings and work the lap some more. Repeat until the rings just clean up. Your lap should be the same size as the scope tube, your sizing, straightening and making the rings as perfectly round as possible while still having a gap between the ring halves to allow those rings to clamp the scope in place. It's not a quick process if a fair amount has to be removed, but I think worth doing if the rifle can shoot well enough to justify it.

Lapping works because the lapping paste or powder needs to embed into a softer metal than the material it's going to be lapping. Aluminum or brass as an example when lapping steel rings. Ideally you'd want to machine the lap from a larger diameter on a lathe that's proven to be capable of turning parallel end to end.
RSG
Posts: 1541
Joined: Fri Feb 04, 2011 9:59 am
Location: Ontario, Canada

Re: Determing the size of a lapping tool

Post by RSG »

chief wrote: Sat Aug 17, 2019 12:42 pm so I thought perhaps it needed to be a few thou undersized.

Terry
No, same size as Pete mentioned. You'll know when the lapping is complete when you've slightly worn all the blueing off both of the entire bottom rings. I wouldn't worry about the top rings.
Vision is not seeing things as they are, but as they will be.
chief
Posts: 236
Joined: Sat Jan 04, 2003 5:51 pm

Re: Determing the size of a lapping tool

Post by chief »

Sorry for a late reply, been away from the cmptr for a while.

Thanks Pete, and RSG, sensable advise from both of you

Pete the rifle is a Savage model 10 and the scope is something I haven't tried before, from Primary Arms, a PA3-18X50 Silver Series. I bought it because it has a 6.5 Creedmoor specific bullet drop reticle that seems to have good reviews on the web, we'll see, I haven't tried it yet.

The rings are no-name rings from Amazon, kind of the reason I wanted to try lapping them.

I'll post back once I turn up the lap and let you all know how it went.

Terry
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